The present invention relates to an electrographic copying machine of the powder image transfer type wherein the magnetic developer recovered by a cleaner is reused for developing latent electrostatic images.
One type of such copying machines which are widely used are machines comprising a magnetic brush developing unit by which latent electrostatic images formed on the surface of an image bearing member are developed with a magnetic developer composed of a magnetic carrier and an insulating toner mixed therewith, i.e. a so-called two-component magnetic developer.
In this type of copying machine, only the insulating toner from the two-component magnetic developer is deposited on the surface of the image bearing member and is used for developing the latent electrostatic image to a toner image. However, part of the toner is wasted and does not contribute to the formation of the copy image since the portion of the toner deposited on the surface of the image bearing member but remaining thereon without being transferred to the copy paper is collected by a cleaner and discarded.
To eliminate the waste of the insulating toner to make its use more efficient, a method has been proposed in which the toner collected by the cleaner is guided into the magnetic brush developing unit and reused for developing latent electrostatic images. While waste of the toner is avoided by this method, the copying machine adapted for carrying out the method has the following problem.
When the insulating toner is collected by the cleaner, dust, including fibers of the copy paper and the like, adhering to the surface of the image bearing member is also collected at the same time, with the result that such dust is led into the developing unit along with the toner. Additionally, the collected toner itself contains toner particles charged with a polarity opposite to the desired polarity, for example, by the influence of the transfer corona charger, and also agglomerates of several toner particles which are fused together. The proportion of agglomerates of toner contained in the collected toner is especially high in copying machines wherein the cleaner has a cleaning blade for scraping the residual toner from the surface of the image bearing member. Although such charged toner particles and agglomerated toner particles are no longer suitable for developing latent electrostatic images, such toner particles are nevertheless led into the developing unit just as is the dust. Accordingly, while the copying machine adapted for carrying out the above method operates longer so as to make an increased number of copies, the dust and the toner particles unsuited for developing latent images (hereinafter referred to collectively as "foreign matter") are accumulated in the developer within the developing unit. Consequently the developer produces marked noise, such as black or white spots or fog, in the copy images formed.
It is therefore essential to overcome the above problem in order to make the foregoing method feasible for use in electrographic copying machines of the powder image transfer type.